The trend in the golf club industry is towards the construction of customized golf clubs. In customizing a golf club, the physical size of the club should correspond in some way to the size of the golfer. For example, a longer golf club would be suitable for a taller golfer. The weight of the club should also be considered since, in general, a golfer with greater physical strength can swing heavier golf clubs than golfers of lesser strength. These are examples of two factors commonly considered when selecting the proper golf club for a particular individual. Another important parameter to consider is the bending characteristic of the golf club shaft.
The bending of a golf club shaft may be characterized by its bending stiffness and its vibrational bending frequency. The bending stiffness is a measure of how much the golf club shaft will bend (i.e. its displacement) due to an applied force at a specified location on the shaft. If the same force is applied in the same way to two different golf club shafts, the shaft with the smaller displacement is considered to be stiffer, as illustrated in FIGS. 1A.
The vibrational bending frequency of a golf club shaft is the frequency at which the golf club shaft vibrates when bent and then suddenly released, for example, when being held at the grip end and deflected at the head. Such vibration of the shaft is similar to the motion of a car radio antenna when struck. As the shaft vibrates, the number of times the end of the shaft goes back and forth each minute is the vibrational bending frequency measured in cycles per minute.
It is common for golf clubs to be purchased pre-assembled as a set, with the golfer required to adapt to the golf clubs as purchased. Some golf clubs may be selected off-the-shelf with a particular stiffness specification that the golfer deems appropriate for his or her golfing style. Golf club shafts are currently commercially sold in different bending stiffness specifications, examples are: “ladies,” “regular,” “stiff,” and “extra stiff.” Each of these specifications relates to a range of bending stiffness values. The exact value of an individual shaft designated with one of the above terms falls somewhere within the range of values described by that specification. The purpose of these various shaft bending stiffnesses is to allow the custom assembly of a golf club with a vibrational bending frequency that best compliments a golfer's particular strength and swing speed
One problem with selecting golf clubs with a fixed bending stiffness and vibrational bending frequency is that it is rare for a golfer's swing tempo to precisely match with an off-the-shelf set of clubs. Another problem is that it is also rare for a set of clubs to have physical parameters such as bending stiffness, mass and vibrational bending frequency consistent between each club within a set.
One solution is to provide a custom-made set of clubs where a golf professional or person with technical expertise consults with the golfer prior to the assembly of the golf club. The consultant chooses the golf club shaft bending stiffness, length and head weight to best suit the individual golfer.
A problem with providing a custom set of clubs is that commonly only a range of discrete vibrational bending frequencies are attainable. Furthermore, the range of discrete vibrational bending frequencies may not be available at all for certain combinations of shaft length and head weight. In addition, once the club is assembled, the vibrational bending frequency and shaft bending stiffness cannot be easily changed without re-manufacturing the golf club.
Some prior golf club shafts are designed to provide very specific shaft bending stiffnesses at different locations along the shaft's length. One prior golf club shaft uses an interior bar within a hollow shaft, and a number of coupling inserts to alter shaft stiffness. When engaged, the coupling inserts attach the shaft to the interior bar, thus increasing the overall stiffness of the club. A problem with such a shaft is that it may provide only minimal stiffness increase due to the inefficient location of the central bar and its contribution to the overall bending moment of inertia. Further, the mass increase of such a device on the overall club mass may be substantial in relation to the stiffness change provided. Such added weight may not be desirable with certain golfers.
Another problem with some prior golf club shafts is that their stiffness may not be altered after they have been assembled by the manufacturer. A golfer's skill level and preferences may change over time and, thereby, the golfer may desire a different stiffness to his clubs. As such, the golfer may be compelled to purchase a new set of clubs or to disassemble and reassemble his/her clubs using new components.
One prior golf club design provides the capability of changing the shaft stiffness of a golf club after it has been assembled. The golf club's stiffness may be changed by pressurizing the shaft with air. One drawback of such a device is that the use of pressure levels necessary to achieve a real benefit may create significant safety issues.
As such, these prior golf clubs do not provide the individual golfer with the capability of changing the bending stiffness and the vibrational bending frequency of a given set of clubs after the clubs have been assembled and purchased without substantially changing the mass of the club or introducing significant safety issues.